MERTIS on its way to Mercury: first hyperspectral observations of the Moon in the thermal infrared

Introduction. The Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) [1] is part of the ESA BepiColombo science payload on its way to Mercury. MERTIS consists of a push-broom IR-spectrometer (-TIS) and a radiometer (-TIR) which operate at 7 to 14 μm and 7 to 40 μm, respectively. The main objectives of MERTIS are to provide the surface composition, global map of the mineralogy, temperature variations and thermal properties of Mercury’s surface [1]. The target spatial resolution of MERTIS-TIS is lower than 300 m at 400 km apoherm and 500 m for the global mapping [1]. The signal-to-noise ratio (S/N) at the Christiansen feature (7.5 μm) is higher than 200 at the day-side temperature of Mercury (450 K - 700 K) [1]. During its cruise, BepiColombo completed an Earth/Moon flyby in 2020. MERTIS was able to acquire data through its space baffle and had the opportunity to observe the Moon during the flyby [1, 2]. Here we present the first hyperspectral observation of the Moon in the thermal infrared wavelengths with MERTIS-TIS.

Data and method. During the flyby, the closest approach occurred on 10^th April 2020 at 4:25 UTC at a distance of around 13 000 km from the Earth. The Moon, in the opposite direction, was at a distance of around 700 000 km. Moon radiation has been recorded through the MERTIS space baffle that was designed to perform deep-space calibration measurements at Mercury [1, 2]. Calibration using deep space, which is the key to obtain the thermal radiation coming from the instrument itself, was obtained before and after the flyby. Data have been calibrated using the internal and external calibration targets of MERTIS: deep space and internal black body at 300K [1]. Due to the spacecraft distance to the Moon, only a part of the MERTIS/TIS detector has been illuminated by the Moon and the resulting spatial resolution is around 500 km/pixel which is far away from the targeted spatial resolution at Mercury (Fig. 1). Observations have been selected in the two main lunar soils: mare and highlands and average spectra for each soil have been derived (Fig. 1). The Christiansen Feature (CF) position is calculated from a parabolic fit in the CF region and the direction of the concavity (c) of the CF is derived as in [3].

Figure 1: MERTIS-TIS pixels on the surface of the Moon (left) used to calculate the average emissivity (right) of mare (green) and highlands (pink).

Results. MERTIS data successfully point to the two major lunar terrains: highlands and mare. The CF position is shifted towards longer wavelengths (around 9.5 μm) in MERTIS data compared to laboratory measurements [4] and DIVINER observations (around 8.25 μm) [3] (Fig. 2). However, the CF of MERTIS-TIS emissivity spectra is shifted toward longer wavelengths in the mare compared to highlands, which is consistent with both laboratory measurements and DIVINER observations. The concavity parameter C is lower in the MERTIS-TIS data, DIVINER data and laboratory measurements in the mare than in highlands (Fig. 2).

Figure 2: (left) Average CF position and C parameter of MERTIS-TIS observations shown in Fig. 1. The error bars correspond to one standard deviation. (middle) CF position and C parameter for the laboratory emissivity [4]. (right) CF position and C parameter of DIVINER data published in [3].

Conclusion. Although the MERTIS-TIS data are shifted toward the longer wavelengths, the differences between mare and highlands are consistent with laboratory measurements and previous DIVINER observations. This wavelength shift is under investigation. MERTIS is able to differentiate the two main lunar terrains despite the spatial resolution and the day-side temperature of the Moon's surface (up to 400 K) lower than that of Mercury, which considerably decreases the S/N. This flyby helps us for the preparation of the future Mercury’s observation and especially the upcoming flybys of Mercury (see Verma et al. and Van den Neucker et al., this conference).

References. [1] Hiesinger H. et al. (2020). Space Science Reviews, 216, 1-37. 54 (11), 1057–1064. [2] Maturilli A. et al. (2020) EPSC2020-271. Copernicus Meetings, 2020. [3] Greenhagen B. T. et al. (2010) Science 329,1507-1509. [4] Barraud, O., et al (2024). LPI Contributions, 3040, 1910.